Recuperators for incinerators

ABSTRACT

A compact, low cost heat recuperator for incinerators and the like having one or more heat exchange tubes with internal and external fins, positioned within an outer tubular shell. Means at one end of the shell hold the heat exchange tubes in fixed position, and an expansion seal is provided at the other. Means are provided for conducting exhaust gases through the heat exchange tubes and process air around the heat exchange tubes for preheating. Connecting means are disclosed for attaching the recuperator assembly to an existing incinerator so that minimum modifications are required.

United States Patent [191 Beck 1 1 Aug. 13, 1974 1 1 RECUPERATORS FORINCINERATORS [75] lnventor: Joseph J. Beck, Berlin, Wis.

[73] Assignee: McQuay-Perfex, Inc., Minneapolis,

Minn.

[22] Filed: Feb. 28, 1973 [21] Appl. No.: 336,436

[52] US. Cl 432/223, 432/255, 110/8 R, 122/D1G. 1, 110/56, 165/179 [51]Int. Cl F28f l/l0 [58] Field of Search 432/223-226,

432/247-249, 255, 22; 110/8 R, 56; 122/DIG. 1; 165/184, 179

[56] References Cited UNITED STATES PATENTS 2,033,685 3/1936 Coutant110/10 2,146,565 2/1939 3,499,400 3/1970 Altmann 110/8 R 3,509,8345/1970 Rosenberg et a1. 110/8 R 3,675,600 7/1972 Jones 110/8 R OTHERPUBLICATIONS lndustrial Furnaces by W. Trink, 4th Edition, Vol. 1,Copyright 1950, pages 374-395.

Primary Examiner-John .1. Camby Assistant ExaminerHenry C. YuenAttorney, Agent, or FirmMerchant, Gould, Smith & Edell 57 ABSTRACT Acompact, low cost heat recuperator for incinerators and the like havingone or more heat exchange tubes with internal and external fins,positioned within an outer tubular shell. Means at one end of the shellhold the heat exchange tubes in fixed position, and an expansion seal isprovided at the other. Means are provided for conducting exhaust gasesthrough the heat exchange tubes and process air around the heat exchangetubes for preheating. Connecting means are disclosed for attaching therecuperator assembly to an existing incinerator so that minimummodifications are required.

1 Claim, 7 Drawing Figures RECUPERATORS FOR INCINERATORS BACKGROUND OFTHE INVENTION This invention relates generally to the field of heatrecuperators for incinerators, furnaces, and the like. Morespecifically, the present invention relates to a compact recuperatoradapted for incorporation into an existing incinerator, requiring aminimum of modifications thereof.

Recuperators are widely used on furnaces in order to increase theirefficiency and reduce fuel cost. Since the heat of the exhaust gasesescaping out the chimney represents a loss, the recuperator is used torecover a good deal of this heat by using the exhaust gases to preheatthe air being fed into the furnace. Less fuel is then required to heatthe preheated air up to the operating temperature of the furnace. In theprior art, many different configurations have been used forrecuperators. Many of these recuperators are very elaborate in designand are very expensive in construction, and this is especially true ofrecuperators designed for large furnaces, where the savings in fuelcosts can be substantial. However, in medium sized and small sizedfurnaces or incinerators, recuperators are often not used because of theadditional initial expense of the recuperator. Another stumbling blockstanding in the way of adding recuperators to existing incinerators isthe fact that many prior art recuperators are integral with theirfurnaces, and would therefore require extensive modification to theincinerator for installation.

The increasing concern of the public over air pollution, and new,tighter laws governing factory emissions have placed new emphasis on theneed for low cost and highly efficient recuperators for moderate sizedincinerators. For example, certain industries such as paint and chemicalindustries are faced with the problem of disposal of noxious fumes orprocess air which are generated as a waste product, and which wereformerly vented to the atmosphere. In many cases, buring of the fumes athigh temperature is the only reasonable method of disposal. A low costand highly efficient recuperator is necessary in order to keep costsdown.

SUMMARY OF THE INVENTION The present invention provides a recuperatorfor an incinerator which is both low in cost and very efficient inoperation. Further, a recuperator according to the present invention canbe added onto an existing incinerator with a minimum of modifications,or can be incorporated into new incinerators with a minimum of redesign.

According to the present invention there is provided a recuperatorassembly for an incinerator, comprising an elongated generallycylindrical outer shell with one or more inner tubular memberspositioned within the outer shell and parallel therewith. The spacebetween the inner tubular members and the outer shell defines a heatingpassage. The tubular member has a plurality of heat exchanging finsprojecting both inside, and outside of the inner tube in the heatingpassage. The shell has an intake port at one end and an outlet port atthe other. Means are provided for connecting the recuperator assembly toan incinerator. Means are provided for connecting the inner tubularmember to receive exhaust gases from the incinerator, and further meansare provided for connecting the outlet port to the air intake of theincinerator, so that process air may be preheated by the exhaust gasesprior to combustion.

In one embodiment, a single heat exchange tube is used, and therelationship in size between the heat exchange tube and the shell issuch that the shell fits snuggly around the external fins of the heatexchange tube so-that process air in the heating passage is forcedaround the fins for efficient heat transfer. In another embodiment, aplurality of heat exchange tubes are used, and are positioned in fixedrelationship within the shell by a tube sheet. One or more baffles arepositioned inside the shell to direct process air flow transversely ofthe heat exchange tubes in this embodiment.

In one method of mounting, a flange at one end of the shell is used forvertical support and attachment of the recuperator assembly on the topof an incinerator. In a horizontal method of mounting the frame supportsone end of the recuperator assembly, while a supporting duct connectsthe other end to the recuperator.

DESCRIPTION OF THE DRAWINGS positioning of a recuperator assemblyaccording to the present invention;

FIG. 6 shows a detail of an expansion joint used in the horizontalmounting embodiment of FIG. 5; and

FIG. 7 is a cross sectional view of a multi-tube recuperator accordingto the present invention.

I DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, reference numeral10 generally designates a recuperator according to the presentinvention. Recuperator 10 is shown mounted upon an incinerator generallydesignated 11. And exhaust stack (not shown) can be positioned on top ofthe recuperator.

The incinerator 11 comprises a housing 13 which defines an innercombustion chamber 14. Intake 15 introduces fumes or process air intothe combustion chamber, and the burned exhaust gases pass upward fromthe combustion chamber through exhaust flu l6. Incinerator 11 alsoincludes a burner assembly 12 for supplying fuel-air mixture to thecombustion chamber, according to the particular design of theincinerator and the type of fuel which is intended to use. At the top ofthe housing 13 of incinerator 11, is a mounting flange 17.

Recuperator 10 comprises an outer shell 20 which forms the housing forthe recuperator. Shell 20' is a generally elongated cylindrical steeltube. Positioned coaxially within shell 20 is an inner, heat exchangetube 21. Since heat exchange tube 21 is in direct contact with the hotexhaust gases, it is preferably made from chrome-moly steel. Heatexchange tube 20 has a plurality of heat exchange fins 22 welded to theinside thereof, and projecting inwardly. Tube 21 also has a plurality ofheat exchange fins 23 welded to the outside thereof. The space betweenshell 20 and heat exchange tube 21 defines a heating passage, asindicated by flow arrow 24. In the single tube embodiment, shown in FIG.1, heat exchange tube 21, fins 23, and shell 20 are sized so that shell20 fits closely around fins 23.

The external fin structure of the heat exchange tube is shown better inFIG. 2. Fins23 are made from a serrated strip which is spirally woundaround and welded to the outer surface of tube 21. The serrations ornotches divide the strip into individual fins. The notches on succeedingturns of the spiral tend to align so as to form a maze of spiral andzigzag passages. Since the outer shell 20 fits snuggly around fins 23,process air passing through the heating passage, in a generallylongitudinal direction to the heat exchange tube, is forced into flowpatterns which create maximum contact between the process air and fins23, thereby helping a maximize efficiency of heat transfer. Since theexternal fins are usually not subjected to high temperatures, the normalmaterial for their construction is carbon steel. In the preferredembodiment, the external fins are approximately one-eighth inch thick byone and a half inch high.

Internal fins 22 are made of lengths of bar stock, which are welded tothe inner wall of tube 21. Fins 22 are welded at an angle and spaced ina spiral pattern so as to impart a spiral flow tothe exhaust gasespassing therethrough. This spiral flow ensures that maximum contact andheat transfer will be realized. Since the internal fins 22 are subjectedto highest temperature and also to corrosion and errosion from theexhaust gases,

, they are preferably made of chrome-moly steel or stainless steel,depending upon the economics and life expectancy of the unit. In thepreferred embodiment, internal fins 22 are approximately one-quarterinch by one inch by three inches.

Due to the plurality of internal fins 22, recuperators according to thepresent invention are not intended for use with incinerators which putout large amounts of dirt or soot, because of the likelihood of foulingor plugging. Rather, recuperators according to the present invention arebetter adapted for use on incinerators for burning off process air ortoxic fumes.

Referring again to FIG. 1, an inlet port 35 is provided near the top ofshell 20. Near the bottom of shell 20 is an outlet port 36. Air admittedto inlet port 35 travels down through the air passages as indicated byflow arrow 24 and exits at outlet port 36. An intake duct 38 connectsfrom port 36 to the incinerator intake 15. An expansion joint isprovided at 37 to allow for dimensional changes caused by heating. Shell20 which is the outer housing of the recuperator is mounted toincinerator 11 by means of a flange 40, and is supported by a pluralityof vertical support braces 41. The details of the mounting are shownmore clearly in FIG. 4.- The inside of incinerator housing 13 is linedwith a refractory material 42 for protection from the heat generatedwithin the combustion chamber 14. Annular flange 17 is welded to the topof housing 14. Another annular flange 40 is welded to the bottom ofshell 20, at 43. Flanges l7 and 40 are held together by bolt and nutassembly 44 at a plurality of locations around the circumferencethereof. Brace 41 helps give the vertical support needed forthestructure.

FIG. 4 also shows the details of the expansion seal generally indicatedby reference numeral 50. Member 51 in conjunction with the upper portionof incinerator housing 13-defines an annular channel around the top ofthe incinerator. The bottom of the heat exchange tube 21 extends looselyinto this channel but is not attached to member 51, so that it canexpand when heated. The seal is a accomplished by a sand packing 52which fills the channel including the space between member 51 and thebottom of heat exchange tube 21.

As shown in FIGS. 1 and 3, a cap 30 is provided at the top of shell 20for fixing the positioning of heat exchange tube 21 and shell 20. Cap 30comprises a flange 54 and a plurality of braces 56. Flange 54 is weldedto heat exchange tube 21, as shown at 47. A flange 45 is welded to thetop of shell 20. A plurality of nut and bolt assemblies 55 hold flanges54 and 45 together. Cap member 30 thus serves to fix the inner tube andouter shell in position with respect to each other. Since they are notfastened together at the bottom end, but rather are sealed by expansionseal 50, the heat exchange tube can be removed by undoing bolt and nutassemblies 55 and lifting tube 21 out the top.

A preferred installation of the heat recuperator assembly of FIG. 1would one in which a portion of the stack of an existing incineratorwould be removed. The existing incinerator would then be equipped with asand seal and flange as shown in FIG. 4, for attaching the recuperator.If the incinerator is not strong enough it would of course be necessaryto give additional support to the recuperator. A-stack can then beattached to the top of heat exchange tube 21 which extends through thetop of cap 30, by any convenient means.

If space would not permit a vertical installation, a horizontalinstallation such as shown in FIG. 5 could be used. In FIG. 5, a singletube recuperator assembly 60 is shown which is substantially identicalto recuperator assembly 10 of FIG. 1. For purposes of clarity, likeelements are given the same reference numerals as in FIG. 1. Horizontalmounting is accomplished through a frame 61 which supports one end ofrecuperator 60,

and a supporting duct 62 which connects the other end of recuperator 20to incinerator 11. Supporting duct 62 I also functions to convey exhaustgases from the incinerator to the heat exchange tube 21. An exhaust duct63 at the other end of recuperator 60 conveys exhaust gases to a stack(not shown). Because it is not feasible to use a sand expansion seal inthe horizontal arrangement, an alternate expansion packing seal is used.In the embodiment of FIG. 5, heat exchange tube 21 and shell 20 arefixed together by an internal extension of flange 64, in a mannersimilar to that shown in FIG. 3. The expansion seal is used at the otherend of the recuperator, as shown in FIG. 6, which is taken along lines6-6 in FIG. 5.

In FIG. 6, heat exchange tube 21 has an annular bar which may be weldedinto place. A flange 71 is welded to the end of shell 20. Flange 71 hasan annular groove 72 in which is placed the expansion packing, which maybe asbestos-graphite material. A retainer ring 73 is fixed to flange 71by nut and bolt assemblies 74. The seal is provided by annular bar andthe expansion packing, which also allows for expansion due todifferential heating of the members.

In FIG. 7, reference numeral 110 generally designates a multiple tuberecuperator assembly according to the present invention. Recuperator 110comprises a shell member 120 which is similar to shell of the embodimentof FIG. 1. A plurality of heat exchange tubes 121 are positioned withinshell 120 and aligned generally parallel therewith. Heat exchange tubes121 have internal and external fins and may be identical to heatexchange tube 21 of FIG. 1. Heat exchange tubes 121 are positioned intoa bundle by upper tube sheet 122 and lower tube sheet 123. These tubesheets have a plurality of apertures for receiving and holding the endsof heat exchange tubes 121, which may be welded in place. Although onlyfour heat exchange tubes are shown in FIG. 7, it will be understood thatany number could be used and that any convenient arrangement such as acircular or rectangular pattern could be used. A pair of baffles 124 and125 are positioned within shell 120 around heat exchange tubes 121,thereby forcing process air entering inlet port 135 to flow generallytransversely of the heat exchange tubes and out outlet port 136. In use,ducts are provided for conveying the process air to port 135, and forconveying the heated process air from port 136 to the intake of theincinerator.

The entire assembly of heat exchange tubes 121, baffles 124 and 125 andupper and lower tube sheets 122 and 123 are fixed in position withinshell 120 at the upper end. A flange 145 is welded around the top ofshell 120. A cap member 130 has a matching flange 154. Tube sheet 122extends between the two flanges and is bolted in place by a plurality ofbolt assemblies. The tubes can therefore be removed by removal of cap130 and lifting the tube assembly out through the top. A sand expansionseal similar to the one shown in FIG. 4 is provided at the lower end ofthe recuperator. A member 151 and the outer wall '13 of the incineratorcooperate to define a channel. A skirt 128 is welded around tube sheet123, and extends downwardly into the channel. Sand packing 52 fills thechannel to make the sea]. A flange 140 is welded around the end of shell120, to match with flange 17 at the top of incinerator 11. A pluralityof bolt assemblies 144 connect the recuperator assembly to theincinerator. The bottom side of tube sheet 123 is covered with arefractory material 127, leaving holes aligned with heat exchange tubes121 for passage of exhaust gases. The refractory material protects thetube sheet from the high temperatures developed in the combustionchamber. I

The use of fins on both surfaces of the heat exchange tube, togetherwith the compact configuration as illustrated in the foregoingparagraphs and drawings, results in a highly efficient and low costrecuperator.

I claim:

1. A recuperator assembly adapted for adding on to an existingincinerator between the main housing and exhaust stack thereof forpreheating process air to be burned therein comprising:

a. a generally cylindrical elongated shell;

b. means attached to the bottom end of said shell for mounting saidshell in vertical position on the main housing of the incinerator overthe exhaust flu thereof;

c. a heat exchange tube having internal and external fins, said internalfins arranged and configured to impart a swirling flow to gases passingthrough said heat exchange tube;

d. means attached to the top end of said shell for mounting said heatexchange tube coaxially within said shell in alignment with the exhaustflue, the inside diameter of said shell being slightly larger than theoutside diameter of said heat exchange tube plus fins, to define aheating passage therebetween;

e. means defining a sand expansion seal around the bottom of said shelland heat exchange tube;

f. a process air inlet port in said shell near the top end thereof;

g. a process air outlet port in said shell near the bottom end thereof;and

h. means for conducting the preheated process air from said outlet portto the intake of the incineratOl'.

1. A recuperator assembly adapted for adding on to an existingincinerator between the main housing and exhaust stack thereof forpreheating process air to be burned therein comprising: a. a generallycylindrical elongated shell; b. means attached to the bottom end of saidshell for mounting said shell in vertical position on the main housingof the incinerator over the exhaust flu thereof; c. a heat exchange tubehaving internal and external fins, said internal fins arranged andconfigured to impart a swirling flow to gases passing through said heatexchange tube; d. means attached to the top end of said shell formounting said heat exchange tube coaxially within said shell inalignment with the exhaust flue, the inside diameter of said shell beingslightly larger than the outside diameter of said heat exchange tubeplus fins, to define a heating passage therebetween; e. means defining asand expansion seal around the bottom of said shell and heat exchangetube; f. a process air inlet port in said shell near the top endthereof; g. a process air outlet port in said shell near the bottom endthereof; and h. means for conducting the preheated process air from saidoutlet port to the intake of the incinerator.